There is a demand for an air compressor for supplying relatively small--sometimes continuous--quantities of compressed air or other gas for services which are more or less mobile such as trucks, busses, mobile homes, etc. where the motive power is derived from the electrical system of a vehicle or other point of non-availability of central power. A compressor for such service can expect little or no service attention, must operate without lubrication, receives no adjustments, and be reliable and efficient. The art has made numerous attempts to utilize the wobbling piston, but designs heretofore have involved requirements of costly and fragile accuracy and which, under the temperatures which may be developed in devices of this character, are not sufficiently rugged to meet the requirements of the market.